The present invention generally relates to echo cancellers, and more particularly to an adaptive echo canceller which includes a coefficient renewing circuit for renewing filter coefficients.
For example, an adaptive echo canceller is used to suppress an echo which is generated at a 2-line/4-line converter of a two-line telephone line. In such an adaptive echo canceller, it is desirable to carry out a tap (filter) coefficient renewal process at a high speed and to reduce the scale of the hardware.
In a two-line telephone line communication system, for example, a phenomenon occurs in which a signal received from the other party returns with a transmitting signal from the receiving station at a 2-line/4-line conversion hybrid transformer when making the 2-line/4-line conversion. The echo canceller suppresses such a phenomenon, that is, the echo.
FIG. 1 shows an example of a conventional adaptive echo canceller. The adaptive echo canceller includes a 2-line/4-line conversion hybrid transformer 1, memories 2 and 5, a pseudo echo generation filter 3, a tap coefficient renewal part 4, a subtracting circuit 6, and a doubletalk detector 7.
The memory 2 stores a reception side input signal Rin. The pseudo echo generation filter 3 generates a pseudo echo by a convolution of tap coefficients (impulse response sequence of an echo path) and the received signal. The tap coefficient renewal part 4 adaptively renews tap coefficients of the pseudo echo generation filter 3. The memory 5 stores the tap coefficients obtained from the tap coefficient renewal part 4 and supplies the tap coefficients to the pseudo echo generation filter 3. The subtracting circuit 6 suppresses an echo within a transmission side input signal Sin by the pseudo echo.
An unwanted signal component of a reception side output signal Rout is supplied to the transmission side as the echo. The function of the adaptive echo canceller is to suppress this unwanted signal component by subtracting from a transmission side input signal Sin the pseudo echo which is generated by the convolution of the estimated impulse response of the echo path and the received signal. The magnitude, time delay and the like of the echo depend on the characteristic of the 2-line/4-line conversion hybrid transformer 1, the kind of cable used, the length of cable used and the like. Furthermore, the magnitude, time delay and the like of the echo also change depending on a change in the temperature and humidity. For this reason, the tap coefficients of the pseudo echo generation filter 3 are generally changed adaptively.
For the sake of convenience, the sequence of the reception side input signal Rin is denoted by X.sub.j, the sequence of the transmission side input signal Sin is denoted by Y.sub.j, the pseudo echo sequence is denoted by Y.sub.j, and the sequence of a transmission side output signal Sout is denoted by e.sub.j. It is assumed that only the echo component is input to the sequence Y.sub.j of the transmission side input signal Sin, and a residual echo is output as the sequence e.sub.j of the transmission side output signal Sout. In the designations used, a subscript j denotes the time. The following formulas (1) through (3) stand when the tap coefficient number (that is, the number of tap coefficients) of the pseudo echo generation filter 3 is denoted by N and an ith tap coefficient at the time j is denoted by h.sub.i.sup.(j), where 0.ltoreq.i.ltoreq.N. ##EQU1##
As described by the formula (2), the pseudo echo Y.sub.j is obtained from the convolution of the tap coefficient h.sub.i of the pseudo echo generation filter 3 and the sequence X.sub.j of the reception side input signal Rin. The tap coefficient h.sub.i is renewed according to the formula (3) every time a sampling is made (at each time) for all values of i. The formula (3) is based on an adaptive algorithm called learning identification method which renews the coefficients so that a square of e.sub.j converges to zero for each sample. The learning identification method yields to a relatively satisfactory convergence characteristic and the hardware realization thereof is simple. For this reason, the learning identification method is most popularly used as the adaptive algorithm of the adaptive echo canceller.
Next, a description will be given of the relationship of the formulas (1) through (3) and the scale of the hardware. The formula (1) includes only one subtraction, but the formula (2) includes N multiplication and addition processes. Furthermore, the formula (3) includes N multiplication and addition processes because all N tap coefficients must be renewed. Accordingly, the scale of the hardware increases proportionally to the tap coefficient number N of the pseudo echo generation filter 3.
A relationship between the suppression quantity of the echo canceller and the renewal of the tap coefficients will now be considered for a case where the formula (3) is calculated with a fixed-point. When storing a calculation result of the right term of the formula (3) into the memory 5, it is necessary to adjust the digits by rounding down certain digits by a truncation. But a truncation error E is generated by this truncation, and this truncation error E is generated only in a negative direction. The truncation error E is accumulated to E.times.m, where m denotes the number of samples. In this case, the tap coefficient is clamped to a negative maximum value and will not converge.
Accordingly, a rounding (rounding off) must be made when renewing the tap coefficient. However, when a narrow band signal is input, the tap coefficient converges differently when compared to the case where other signals are input. Hence, when the rounding is made for each sample, a rounding error is gradually accumulated, and there are problems in that the characteristic of the adaptive echo canceller is greatly deteriorated and a stable operation cannot be carried out. Therefore, in order to ensure a stable operation of the adaptive echo canceller regardless of the kind of signal input, it is necessary to carry out an extremely complex process of making the rounding and sometimes making the truncation.
FIG. 2 shows a conventional circuit part for selectively making the rounding and truncation. The circuit part shown in FIG. 2 includes the tap coefficient renewal part 4, the memory 5 and the doubletalk detector 7 which are connected as shown. The tap coefficient renewal part 4 includes a rounding process part 41, a truncation process part 42, a controller 43, a tap coefficient renewal circuit 44, and switches 45 and 46. Normally, the switches 45 and 46 are connected to the rounding process part 41 under the control of the controller 43. However, the switches 45 and 46 are switched and connected to the truncation process part 42 with a period corresponding to a predetermined number of samples under the control of the controller 43.
In the adaptive echo canceller, the renewal of the tap coefficients must be made within one sampling period for all of the tap coefficients. Suppose that the adaptive echo canceller is realized by a digital signal processor. The digital signal processor is generally provided with a function of truncating the calculated result which is obtained in the tap coefficient renewal part 4 when storing the calculated result into the memory 5, and the time required to carry out this truncation process is short. On the other hand, it takes time to carry out the rounding process by a software process, and a long time is required to carry out the rounding process for all of the tap coefficients. The adaptive echo canceller which has the tap coefficient renewal part 4 shown in FIG. 2 must carry out the rounding process within one sampling period, but when the rounding process cannot be completed within one sampling period, it becomes necessary to use a plurality of digital signal processors and reduce the processing time by a parallel processing. However, the use of the plurality of digital signal processors will increase the scale of the hardware.
Therefore, in order to ensure a stable operation of the adaptive echo canceller with respect to any kind of input signal including a narrow band signal when renewing the tap coefficients of the pseudo echo generation filter 3 by the fixed-point operation, it is essential to switch the process between the rounding and the truncation. However, there are problems in that the switching between the rounding and truncation processes requires a complex control by the controller 43 and the scale of the hardware inevitably increases.